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Cosmopolitan <i>Acrosorium</i> species with hook-forming thalli have been merged under the name of <i>Acrosorium ciliolatum</i> (Harvey) Kylin through a long and complicated nomenclatural history. We examined the specimens of ‘<i>A. ciliolatum’</i> and related taxa from the northwestern (NW) Pacific, the UK, southern Spain, Australia, New Zealand, and Chile, using morphological and molecular analyses. We confirmed that these specimens are separated into four clades based on <i>rbc</i>L phylogeny, and the absence or presence of terminal hook-like structures represent intraspecific variation. Our results indicated that <i>Acrosorium flabellatum</i> Yamada, <i>Cryptopleura hayamensis</i> Yamada, <i>Cryptopleura membranacea</i> Yamada and the entities known as ‘<i>A. ciliolatum’</i> in the NW Pacific are conspecific; the name <i>A. flabellatum</i> is the oldest and has priority. This taxon exhibits extreme variations in external blade morphology. We also confirmed that the position of the tetrasporangial sori is a valuable diagnostic characteristic for distinguishing <i>A. flabellatum</i> in the NW Pacific. We also discussed the need for further study of European and southern hemisphere specimens from type localities, as well as the ambiguous position of California specimens.